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Abstract

Background:Autophagy is an evolutionarily conserved process of protein and organelle recycling. Under basal conditions, autophagy is critical for protein and organelle quality control. This cannibalization mechanism, however, can be detrimental under certain conditions, and dysregulation of autophagy has been implicated in numerous diseases. Recently, activation of autophagic flux has been reported in cardiac hypertrophy, heart failure, myocardial infarction, and ischemia/reperfusion injury. The unfolded protein response (UPR) is a cellular mechanism triggered by folding stress in the ER. When protein folding capacity, governed by ER resident chaperones, is overwhelmed by misfolded proteins, ER stress ensues, stimulating chaperone protein expression, ER associated degradation, and ultimately cell death if the stress is not remediated. Recent studies in yeast suggest the UPR can directly activate autophagy by phosphorylating ATG1, a critical upstream kinase required during autophagy initiation. However, whether and how ER stress, which is active in cardiac disease, regulates autophagy in heart is unknown.

Methods and Results:Using neonatal rat ventricular cardiomyocytes in culture, we found the classical ER stress inducer, tunicamycin, triggers profound UPR signaling and autophagy up-regulation. The processing of LC3-II, an indication of autophagy activity, is dramatically increased. As multiple pathways are involved in ER stress, we focused on the IRE1/XBP1 branch. With cardiomyocyte-specific over-expression by lentivirus in vitro, we observed robust activation of autophagy. Further, we found that in vivo over-expression of XBP1s in cardiomyocytes triggered autophagy, as evidenced by real-time PCR and immunoblotting assays. As autophagy markers can accumulate due to blockage of lysosomal degradation, we quantified the lysosomal proteins, cathepsin D and LAMP1, finding each to be increased, suggesting that autophagic activity and flux per se are enhanced.